This invention relates to heterogeneous compositions comprising fluoroelastomers and unsintered porous particles of polytetrafluoroethylene, and methods for manufacturing such compositions and devices made from the same.
Fluoroplastics, such as polytetrafluoroethylene ("PTFE"), are suited to a variety of uses because of their chemical inertness and desirable physical properties. Expanded fluoroplastics are characterized by a unique microporous node and fibril structure, see U.S. Pat. No. 3,953,566, Apr. 27, 1976. These expanded fluoroplastics are useful for making porous, flexible articles, including implantable prosthesis for humans and animals. Such porosity is highly desirable in devices such as vascular grafts, for example, because it promotes tissue ingrowth. The use of expanded PTFE for the fabrication of porous prosthetic implants has been somewhat problematic because of the considerable difficulty in maintaining the porosity while providing the implant with adequate strength.
Some such processes involve adding fillers to the PTFE prior to forming the prosthesis. This filler may be retained in the prosthesis, or subsequently removed by, for example, leaching it out of the formed article with a solvent to provide for the desired porosity. In either event, the use of fillers has not meet with much success. The chemical inertness of fluoroplastics, such as PTFE, although a very desirable property, is disadvantageous since many materials are not compatible or miscible with PTFE. Thus blending fillers with PTFE can be difficult. The incomplete blending of the fillers with the PTFE may cause, for example, failure of the resulting prosthesis by delamination along the contact areas between the filler and the PTFE.
Fluoropolymers possess elastomeric properties, i.e. fluoroelastomers, are also difficult to process due to their high molecular weight and melt viscosity. Elastomers, which are useful for manufacturing many types of devices because of their unique physical properties, are more easily processible by thermoplastic extrusion techniques than fluoropolymers, even fluoroelastomers which share many of the same physical properties. Fluoroelastomers, including poly-(tetrafluoroethylene-co-propylene), disclosed in U.S. Pat. No. 4,463,144, have a high molecular weight, and hence can only be extruded with difficulty. Fluoroelastomers suffer the same disadvantage associated with processing fluoropolymers in generally due to their high molecular weight and melt viscosity.
Attention has recently been concentrated on preparing fluoropolymers of copolymers of tetrafluoroethylene (TFE) and various unsaturated monomers, such as propylene. These novel fluoroelastomers take advantage of the known heat and chemical resistance of TFE, while obtaining a more easily processible polymer. Such copolymers are disclosed in U.S. Pat. No. 2,468,664 and British Patent No. 594,249.
The specific copolymers exemplified in the aforementioned patents, and the properties disclosed therein indicate that when about one-half or more of the repeating units of the copolymers are residues of tetrafluoroethylene, the resultant copolymers possess characteristics of fluoropolymers. That is, such copolymers are tough, non resilient, high melting point plastics. When the tetrafluoroethylene units do not predominate in the copolymer that the resulting copolymers are similar in properties to low melting point thermoplastic resins.
However, the above discussed fluoropolymers, and specifically fluoroelastomers, are not useful materials, in and of themselves, for preparing vascular grafts. These types of polymers do not possess the same strength, or porosity required for the preparation of vascular grafts. In this regard, none of the discussed polymers, even fluoroelastomers, when molded into a vascular graft possess the necessary porous-microstructure of pure PTFE. It would thus be desirable to have a polymer, or composition of polymers, which exhibit the desired porosity of PTFE, while also exhibiting the strength and elasticity of elastomers in general.
U.S. patent application Ser. No. 892,271, filed Aug. 4, 1986, and assigned to the assignee of this application, teaches a blend of PTFE and fluoroelastomers. The taught blend incorporates the fluoroelastomer into the PTFE matrix, with the elastomer coating the individual PTFE fibrils. This blend is used to prepare vascular grafts. Other patents teach the blending of PTFE and various fluoroelastomers, See U.S. Pat. Nos. 4,596,839, issued to Peters on June 24, 1986; 4,507,439, issued to Stewart on Mar. 26, 1985; 4,387,168, issued to Morita on June 7, 1983; 4,568,716, issued to Derencsenyi on Feb. 4, 1986 and 4,555,543, issued to Effenberger et al on Nov. 26, 1985. The compositions taught in these various reference include other additives for enhancing the compatibility between the fluoroelastomer and PTFE.
While the above discussed references provide for compositions which are a blend of the PTFE and the fluoroelastomer it remains highly desirable to form a composition of a fluoroelastomer which is loaded with PTFE. This composition would exhibit the beneficial properties of fluoroelastomers, while having the improvement of the strengthening property of PTFE.